Typed return (GLib.Value).
Unset (GLib.Type.INVALID) is omitted on the wire. A number or string uses the same Bin.StreamValue encoding as args elements. A GObject is one object. A Gee.ArrayList is an object array. Live GObjects write a lease id when Transport.Connection.live_handles is on.
var n = GLib.Value(typeof(int));
n.set_int(3);
resp.retval = n;